This invention relates to extruders of the type in which a screw rotatable within a barrel is employed to extrude material to a die or injection mold connected to the outlet end of the barrel. The invention is concerned particularly with improvements in high output plasticating extruders. This is an improvement on the invention described in my U.S. Pat. No. 4,173,417 issued Nov. 6, 1979.
A plasticating extruder receives polymer pellets or powder (often together with formulation additives in liquid or particle form), works and raises the temperature of the polymer sufficiently to dispose it in a melted or plastic state, and delivers the melted polymer under pressure through a restricted outlet or die. Ordinarily, it is desirable that the discharge extrudate be fully melted, well mixed, uniform in temperature and pressure, and substantially free of small gels and other fine structure agglomerations. It is also desirable that the rate of delivery of the molten polymer through the die be regulated simply by changing the rate of extruder screw rotation and that the rate of delivery at the selected screw speed be substantially uniform.
The basic extruder apparatus includes an elongated barrel which may be heated or cooled at various locations along its length and a screw which extends longitudinally through the barrel. The screw has a helical conveying land on its surface which cooperates with the cylindrical internal surface of the barrel to define an elongated helical channel.
An extruder screw ordinarily has a plurality of sections which are configurations specifically suited to the attainment of particular functions. Examples are "feed" sections and "metering" sections, which are of basic importance and are present in nearly all plasticating extruders for handling thermoplastic polymers.
A typical extruder screw feed section extends beneath and forwardly from a feed opening where polymer in pellet or powder form is introduced into the extruder to be carried forward along the inside of the barrel by the feed section of the screw. As the material is advanced along the channel, it is worked. This, in turn, generates heat, and melting of the polymer occurs as the material is moved along the feed section and later sections of the screw. Actually, most of the melting typically occurs near the surface of the barrel at the interface between a thin melt film and a solid bed of polymer. This general pattern persists until a substantial portion of the polymer reaches the melted state. It is usually advantageous to employ a tapered transition section between the relatively deep feed section and the shallower metering section. Prior to solid bed breakup, this keeps the solid bed width larger and more tightly pressed against the barrel wall, thereby enhancing the melting rate. After some 40% to 70% of the polymer has been melted, solid bed breakup usually occurs, and at this time particles of solid polymer become disbursed in the polymer melt.
An extruder screw metering section has as its special function the exertion of a pumping action on the molten polymer. Ordinarily, the throughput achieved by a screw is thought of as being a function of the combination of the "drag flow" and "pressure flow" effects of the metering section.
Drag flow is basically the flow which results from the relative movement between the screw and the internal surface of the extruder barrel. It may be thought of as being proportional to the product of the average relative velocity and the channel cross-sectional area. Stated in another way, the drag flow is the volumetric pumping capacity, the latter being a function only of screw channel dimensions times screw rpm. This drag flow component is directed toward the outlet end of the screw. It may be increased by increasing the speed of the screw and/or by increasing the depth of the flow channel in the screw.
Acting in opposition to drag flow is a pressure flow component stemming from the reluctance of the material to flow through the restricted outlet opening at the end of the extruder passage. The speed of the screw does not directly affect the pressure flow component but of course it may affect such factors as back pressure and material viscosity, which factors, in turn, affect significantly the pressure flow component. On the other hand, pressure flow is directly affected by both the depth and length of the screw channel; an increase in channel depth has a tendency to increase greatly the pressure flow component and an increase in channel length has a tendency to reduce this back flow component.
In order to better understand the benefits of the present invention, it is helpful to briefly consider recent developments in the art aimed at optimizing extrudate production. In particular, developments have been aimed at increasing the rate of mixing and circulating melt and solid material.
Traditionally, melting was believed to be most efficient when it can be made to follow the "Tadmor Melting Model", which requires the solid bed to remain unbroken in the front of the channel, while a melt pool collects in the back. In this model, most of the melting takes place in a thin melt film between the solid bed (rotating with the screw) and the stationary barrel.
For conventional screws, the solid bed exists for much of the melting distance, as can be demonstrated by a technique often referred to as "screw pushouts". The technique involves bringing an extrusion operation to steady state condition and then abruptly stopping the process. Cooling water is then immediately circulated on the outside barrel wall and inside the full length of the screw. Later, the barrel wall is heated a short time slightly above the polymer melting point. Finally, the screw is pushed out of the barrel. Then, at every turn of the screw, the contents of the channel are cross-sectioned, revealing visually the distribution of solid particles and melt.
As a result of published screw pushout pictures and interpretations, mathematical melting equations have been developed to serve as a basis for computer programs by Tadmor and Klein and others to simulate screw performance. According to well-known publications on extrusion processing and computer simulation, the ideal screw was supposedly obtained by computer simulation of performance within various combinations of feed, meter, and transition section lengths, depths, pitch, diameter, and operating conditions. Solid bed breakup was considered very bad; therefore, the conventional wisdom was to minimize solid bed breakup until melting was almost complete.
Subsequently, it has been recognized that solid bed breakup for single screw extruders is often highly beneficial. By performing lab extrusions with frequent "pushouts" and comparing the results with computer simulations, it has been noted that examples of early solid bed breakup for some screws show favorable results when subjected to favorable mixing conditions.
Concurrently, it was recognized that a very high rate of melting under these favorable mixing conditions occurs only after melting is about 40% complete, and in the quick melting region, the solid bed is extremely broken up.
Other studies have utilized a revised version of the original Tadmor and Klein computer program to predict the correct melting length of a melt decompression screw. At the shallow downstream end of a tapered compression section, the channel depth of the meter section is suddenly made much deeper in a melt decompression screw similar to the downstream end of a single wave "bump". However, the computer program computed a drastically different percent of melt at the "bump" than observed in the screw pushouts. The pushouts revealed much slower rates of melting in the tapered transition followed by an extremely fast rate of melting and widely disorganized solids distribution in the vicinity of the "bump".
Subsequently, a pushout from a two stage vented polypropylene screw was examined. It showed melting up to the end of the metering section, followed by a much faster rate of melting in the decompressed vent area. This result was unexplainable, even by computer programs.
It was determined to not rely on the metering section design alone to estimate output except where the section is sufficiently long to dominate the pumping characteristic. In general it takes at least several constant depth turns in the metering section channel to strongly influence the output. Thus, it is beneficial to replace the whole length of a metering section with repetitive cyclic waves, in which each repeating wave cycle reinforces the pumping characteristic of the first in the same manner as each turn at constant depth would reinforce the first.
This led to the single channel wave design described in U.S. Pat. No. 3,870,284, where the constant depth metering section was replaced with repetitive cyclic waves. By utilizing small diameter single wave screws, compression at the high wave crests followed by mixing after each decompression increases the melting rate and promotes heat transfer from the melt to the solid. Because of efficient melting, decreased pressure surging and increased rates of production result.
One problem especially for larger diameter single channel wave screws is that solids may obstruct passage at the wave crests. To solve this problem, a central undercut barrier between out-of-phase cyclic waves has been used to create two helical channels so that a solid-rich fraction of the polymer "jams up" into the wave crest while a melt-rich fraction escapes into the deep channel region across the undercut barrier. Such a design assures greater pressure and output uniformity because at the approach to each wave crest, the melt hydraulic pressure is in direct communication with a deep channel section across the undercut central barrier. Furthermore, there is more sideways mixing and less restrictive wave crest squeezing.
It has also been established that a twin channel wave meter section can be preceded by a solids/melt separation design in the transition section to enhance additional melting prior to entering the double wave. A single or twin channel wave can be used in the transition section before entering the metering section, but the value of the double wave principle has been questioned until melting is at least 40% complete. It has been noted that a twin channel wave followed by a mixing tip can withstand much higher extrusion rates than a conventional screw with mixing tip. This is partly because at high speeds, the twin channel wave still melts most, if not all, of the throughput.
In the above-referenced patents there is described an extruder screw whose metering section includes one or more channels following a wave-like cyclical pattern wherein each channel includes periodic wave peaks. The wave portion of the screw performs both metering and mixing functions. Insofar as metering or pumping is concerned, the cyclic wave pattern functions like conventional long metering sections of constant depth in the sense of providing uniform output approximately proportioned to screw rotational speed and providing normal resistance to pressure flow in a rearward direction along the screw channel.
In addition to its good metering properties, the wave portion of the screw has the advantage of achieving good mixing of the polymer without generating excessive heat. In regions of the wave crests, the material is subject to relatively high shear forces so that incompletely melted polymer will be worked and mixed vigorously with the molten material. The material passes from each zone of high shearing action into an adjacent zone of increased channel depth where the heat generating effects are much less intense.
The twin channel wave screw design with a constant pitch undercut barrier has performed effectively to produce high quality melt. Notwithstanding the successful performance, efforts are continuously directed toward increasing the rate of extrudate production.
It has been heretofore proposed in U.S. Pat. No. 4,215,978, issued Aug. 5, 1980 to Takayama et al, in an effort to intensify melting and mixing of extrudate, to provide a plurality of barriers, each crossing a dividing line extending along the screw flight, wherein the dividing line divides the bottom surface of the valley into a plurality of portions along the screw flight. The plurality of barriers form a dam between one hill and the opposite hill at the bottom of the screw for preventing intercommunication of the valleys of the divided portions. In addition, each barrier has one edge line extending from one hill and terminating in the hill side of the opposite hill to form one cross point. The barriers are inclined at an angle with respect to the screw flight such that one of the angularly adjacent dams lies in a mutually intersecting direction with the other. Proposals such as this do not effectively deal with the problems of overheating, especially due to the rather long wave peaks created by the barriers. In addition, such proposals do not achieve the metering output stability that is commonly experienced in twin channel wave screws.